A hush kit is a device for reducing noise from an engine. It is typically used on a low-bypass turbofan engine in older commercial aircraft.

Jet aircraft generate a lot of noise, contributing to noise pollution near airports. In modern high-bypass turbofan engines, the fan mounted afore the jet engine core is very large. The bigger the fan in comparison to the jet core, the more effective the bypass air is in enveloping the jet exhaust at the rear of the engine, reducing noise. The larger turbines needed to spin the large fan slow the jet exhaust, which also reduces noise. A hush kit produces a similar effect, using several modifications to the existing engine. Primarily, a device called a multilobe exhaust mixer on the rear of the engine mixes the exhaust gases of the jet core with the surrounding air and the small amount of bypass air available. Similar systems are also employed on many modern turbofan engines as standard equipment to further reduce noise. Most kits also make further modifications to the exhaust via acoustically treated tailpipes, revised inlet nacelles and guide vanes, all of which reduce forward propagating high-pitched noise caused by the small, high-speed fan.

Modern aircraft equipped with high-bypass turbofan engines are able to comply with contemporary aviation noise abatement laws and ICAO regulations. Hush kits are used on the many older freight and passenger aircraft still in service, such as the Boeing 727 and 737-200, Douglas DC-8 and DC-9, and Tupolev Tu-154. While hushkits are normally found on older aircraft, small business jets and other aircraft too small to be fitted with large, high bypass turbofan engines will be manufactured with hushkits installed, a more economical way to meet noise restrictions than expensive engine or design changes.

Hush kits add weight to aircraft that are fitted with them. They sometimes reduce engine efficiency, which can lower aircraft range and/or cruise speed. For example, a hush kit for the Gulfstream II might weigh 106 kilograms (234 lb) (total airplane weight 29,700 kg (65,500 lb)) and cause about a 1.6% reduction in aircraft range due to the hush kit's reduction in engine efficiency.[1] Another hush kit for the Gulfstream G-II and G-III jets weighs about 170 kg (370 lb) and has a calculated 2% drop in range, although the reduction in range has not been noticed during actual flights.[2] A different example is Federal Express's hush kit for its Boeing 727s, which adds 410 kg (900 lb) of extra weight (total airplane weight up to 86,000 kg (190,000 lb)), causing a 0.5% increase in fuel burn for short trips and no measurable increase for long flights.[3][4]

While hush kits effectively reduce noise emissions from older aircraft, noise cannot always be reduced to the level of modern planes at a reasonable cost. In 1999 this has led to a dispute between the United States and the European Union. The EU's proposed noise ordinances effectively prevented the use of hush-kit outfitted aircraft in Europe, reducing the value of the mostly American used airplanes so equipped and hurting the profits of American hush kit manufacturers. EU Regulation 925/99 was passed over US threats to ban Concorde, but was superseded and effectively repealed by EU Directive No. 2002/30/EC issued March 26, 2002.